The three somatotypes — endomorphic, mesomorphic, and ectomorphic — are basic classifications of animal body types according to the prominence of different basic tissues types, roughly: digestive, muscular, and nervous tissues. They form the core of a theory, developed in the 1940s by American psychologistWilliam Sheldon, associating body types with human temperament types. This linkage is fairly simplistic and is seen as outdated in physiological science, but the account of somatotypes is still probably a valid, if limited way to sort basic body types.

Ectomorphic body type (sometimes referred to as the "banana") is characterized by long arms and legs and a short upper body, high forehead, slightly narrow shoulders, and supposedly have a higher proportion of nervous tissue. They also have long and thin muscles. Ectomorphs usually have a very low fat storage; therefore they are usually referred to as slim. Famous ectomorphs include Joey Ramone, Angelina Jolie, Brad Pitt, Johnny Depp, Kate Moss, Doug Jones, footballer Peter Crouch and swimmer Mark Foster.

Mesomorphic body type (sometimes referred to as "hourglass" for a female and "rectangle" for a male) is characterized by a high rate of muscle growth and a higher proportion of muscular tissue. They have large bones, solid torso combined with low fat levels. It is also noted that they have wide shoulders with a narrow waist. Famous mesomorphs include Mariah Carey, Oprah Winfrey, Bruce Willis and Alicia Keys.

Endomorphic body type (sometimes referred as "apple" for those who put most muscle and fat on the top half of the torso, and "pear" for those who put most muscle and fat on the bottom half of the torso) is characterized by an increased amount of fat storage, due to having a larger number of fat cells than the average person, as well as higher proportion of digestive tissue. They have a wide waist and a large bone structure. Famous endomorphs include Sarah Michelle Gellar (pear), Jack Black (apple) or Meat Loaf (apple)

In his 1954 book, Atlas of Men, Sheldon categorised all possible body types according to a scale ranging from 1 to 7 for each of the somatotypes, where the pure endomorph is 7–1–1, the pure mesomorph 1–7–1 and the pure ectomorph scores 1–1–7. From type number, an individual’s mental characteristics could supposedly be predicted. Sheldon's research showed that a predisposition towards criminality might be influenced by a somatotype high in endomorphy and intermediate in mesomorphy, and in contrast, a predisposition towards suicidality might be influenced by a somatotype high in ectomorphy; on the other hand, ectomorphs were found to be more common in mental institutions.

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Some of this is useful in general and human biology, if weak behavioral science. Advanced triploblastic animals, such as mammals, or modern humans in particular, do have these three basic tissue layers. Sheldon himself was more a behavioral psychologist than either an anatomist or a physiologist. His behavioral conclusions were based largely on interviews which he or his students carried out over a long span of time, and the actual psychometric data was often more suggestive than conclusive. The physical traits are still a useful way of defining body types, and filtered of these conclusions, may provide a base-line for future research.

The factual accuracy of this article is disputed. Please see the relevant discussion on the talk page.

The three body type descriptions could be explained as differences in body composition, which can be altered by specific diets and training techniques.

After a period of significant weight loss, a person who was once considered an endomorph may begin to instead resemble an ectomorph. Likewise, an athletic mesomorph may begin to look more like an endomorph as he ages and loses muscle mass.

However, some aspects of the Somatotype cannot be changed, for example, a characteristic of an endomorphic body type is wide shoulders as well as a wide waist. Ectomorphs may have narrower shoulders and slimmer waists. Although, muscle and adipose mass can be changed, the bone structures associated with endomorphs, ectomorphs and mesomorphs means completely changing between two somatotypes is not always possible.

There is evidence that people with different physiques are targeted by cultural stereotypes. For example, one study found that endomorphs are likely to be perceived as slow, sloppy, and lazy. Mesomorphs, in contrast, are typically stereotyped as popular and hardworking, whereas ectomorphs are often viewed as intelligent but fearful.[1] Stereotypes of mesomorphs are generally much more favorable than those of endomorphs. Stereotypes of ectomorphs are somewhat mixed.

The majority of scientists today generally consider these theories from the 1930s and 1940s outdated. Sheldon's theories had popularity through the 1950s. Some saw in the somatotypes a link to eugenics and racial hygiene. Sheldon's 4,000 photographs of naked Yale undergraduates and other similar photographs were destroyed.[2][3] The words endomorphic, mesomorphic and ectomorphic are still sometimes used to describe body types, maybe especially in association with weight training aimed at gaining muscle. In some types of New Age there is an interest in this kind of correlation between physiology and psyche.

Iwawaki, S., & Lerner, R. M. (1974). Cross-cultural analyses of body-behavior relations: I. A comparison of body build stereotypes of Japanese and American males and females: Psychologia: An International Journal of Psychology in the Orient Vol 17(2) Jun 1974, 75-81.

Iwawaki, S., & Lerner, R. M. (1976). Cross-cultural analyses of body-behavior relations: III. Developmental intra- and inter-cultural factor congruence in the body build stereotypes of Japanese and American males and females: Psychologia: An International Journal of Psychology in the Orient Vol 19(2) Jun 1976, 67-76.

Mouret, M., Revest, L., & Castel, H. (1976). Contribution to the study of physical representation of a school master by pupils 7 to 14 years old: The making of a stereotype: Cahiers de Psychologie Vol 19(1) 1976, 71-85.